Treatment of porous article

ABSTRACT

The present invention is a method for modifying at least one property of a porous membrane. The method comprises the steps of providing a porous membrane. The method also includes exposing the membrane to a fluid at supercritical conditions. At least one property of the membrane is modified while the membrane is exposed to the fluid at supercritical conditions. The condition of the fluid is changed in such a manner that the porous membrane retains the modified property. The present invention is also sheet material that is water-resistant, moisture vapor transmissive and air permeable. The sheet material comprises a membrane having an open pore structure including surfaces defining a plurality of interconnecting pores extending through the membrane and between major sides of said membrane in which the pores have an average pore size. The membrane is made from a material tending to absorb oils and contaminating agents. A uniform coating of precipitated fluorinated urethane polymer material on at least portions of the surfaces of the nodes and fibrils defining the pores. The precipitated fluorinated urethane polymer material provides oil and contaminating agent resistance of at least a number 6 by AATCC 118 testing and an air permeability of at least 0.20 CFM per square foot by ASTM D737 testing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.10/255,043, filed on Sep. 20, 2002, which is hereby incorporated hereinby reference.]

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to treating a porous article andto the resulting treated article. In particular, the present inventionrelates to treating a porous membrane to modify one or more property orcharacteristic of the membrane and to the membrane with the modifiedproperty or characteristic.

2. Description of Related Art

It is known that a porous membrane may have at least one property thatis limited by the material that the membrane is made from. For example,a porous membrane made from an expanded polytetrafluoroethylene (ePTFE)material that is intended for use in garments and apparel has excellenthydrophobicity so it is considered to be waterproof at relatively lowchallenge pressure. However, the ePTFE membrane tends to absorb oil.Such a tendency to absorb oil could affect the hydrophobicity in thearea of the membrane that has absorbed the oil so that area of themembrane may no longer be considered waterproof.

U.S. Pat. No. 4,194,041 discloses a way to protect an ePTFE membranefrom contamination by oil. A continuous hydrophilic film is attached tothe ePTFE membrane to protect one side of the ePTFE membrane from oil.This structure is not air permeable and the hydrophilic film mustcontain moisture to transmit the moisture through the membrane. Aheavier garment results from the necessary moisture present in thehydrophilic film. A person wearing a garment incorporating the membranewith the hydrophilic film often can feel uncomfortable because thehydrophilic film that contains moisture contacts the wearer's body,especially in cool environments. Such discomfort has been described as a“wet and clammy” feeling. This discomfort may be further aggravated by alack of air moving through the garment that could serve to carry themoisture away from inside the garment.

U.S. Pat. No. 5,539,072 discloses the use of relatively smallfluorinated acrylate particles to form a protective coating on amembrane. U.S. Pat. No. 5,976,380 discloses using a solution to providea hydrophilic coating on a porous membrane. U.S. Pat. No. 5,156,780discloses the in-situ polymerization of a protective coating layer onmembrane.

U.S. Pat. Nos. 6,228,447 and 6,410,084 disclose an improved membranestructure that is air permeable to overcome the discomfort drawbackdescribed above yet protect the ePTFE membrane from oil contamination. Afluorinated acrylate oleophobic treatment is applied from relativelylarge particles in an aqueous dispersion in a manner so pores in theePTFE membrane are not completely blocked. Air flow is permitted throughthe ePTFE membrane while it is protected from oil contamination. Theeffectiveness of the treatment is dependent on the particle size of thetreatment material relative to the effective pore size in the ePTFEmembrane.

Alternative and improved treatment methods and treatment materials aresought to minimize this dependency on the treatment material particlesize relative to the pore size to be treated. It is also desired thatmodified properties or characteristics of the membrane be provided insome cases other than oleophobicity. These properties may includesintering, hydrophilicity, electrical conductivity, ion conductivity,porosity, optical reflectivity and color.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a method for modifying at least oneproperty of a porous membrane. The method comprises the steps ofproviding a porous membrane and exposing the membrane to a fluid atsupercritical conditions. At least one property of the membrane ismodified while the membrane is exposed to the fluid at supercriticalconditions. The condition of the fluid is changed in such a manner thatthe porous membrane retains the modified property.

The method further includes the steps of providing a treatment materialthat is soluble in the fluid at the supercritical conditions. The porousmembrane is exposed to the treatment material dissolved in thesupercritical fluid for a predetermined amount of time and at apredetermined temperature and pressure. The treatment materialprecipitates onto surfaces of the porous membrane to effect themodification of the property of the porous membrane when the fluidcondition changes to a state in which the treatment material is nolonger soluble.

The method includes the step of providing a fluid that has a surfacetension less than 5.0 dynes/cm. The method also includes the step ofproviding carbon dioxide (CO2) as the fluid. The providing carbondioxide as the fluid step may further include the step of providing aco-solvent to aid in solubilizing the treatment material in the fluid.The property of the membrane that is modified is selected from the groupcomprising the amount of amorphous content, porosity, oleophobicity,hydrophilicity, electrical conductivity, optical reflectivity, ionconductivity and color.

The method also includes providing an open pore membrane. The providingan open pore membrane step includes providing an expandedpolytetrafluoroethylene (ePTFE) membrane. Fluid flows through more thanone layer of porous membrane in a plurality of layers wrapped on aperforated core.

The method may include the step of exposing the PTFE material of theePTFE membrane to carbon dioxide (CO2) at supercritical conditions toswell a portion of the PTFE material from an initial size to a swelledsize. Crystalline bonds in the swelled portion of the PTFE materialbreak to render the swelled portion more amorphous. Exposure of thecarbon dioxide (CO2) at supercritical conditions to the PTFE material isremoved so the portion of the PTFE material returns towards the initialsize while retaining the more amorphous condition in that portion of thePTFE material.

The method may also include the step of retaining a portion of thetreatment material in a portion of the ePTFE membrane by moving aportion of the treatment material dissolved in supercritical carbondioxide into a swelled portion of the PTFE material. The PTFE materialis permitted to return towards its original size and configuration toabsorb the portion of the treatment material within the PTFE material asthe exposure to supercritical carbon dioxide is removed. The absorbedportion of the treatment material may exude from the PTFE material.

The present invention is also directed to the membrane made according tothe method of the present invention that is waterproof, moisture vaportransmissive and air permeable. The membrane has a structure defining aplurality of pores extending through and between the major sides of thesheet material. A substantially uniform fluorinated urethane polymercoating is deposited on surfaces of the membrane without completelyblocking the pores in the membrane. The coating modifies at least oneproperty of the membrane, such as oleophobicity.

The coating is applied in a low surface tension solution capable ofentering the pores in the membrane. The coating is deposited on surfacesof the nodes and fibrils upon rendering the coating insoluble in thesolvent. The solvent is carbon dioxide in a supercritical state.

The precipitated fluorinated urethane polymer material provides oilresistance of at least a number 6 per AATCC 118 testing while permittingan air permeability of at least 0.20 CFM per square foot by ASTM D737testing. At least a portion of the fluorinated urethane polymer isabsorbed by amorphous portions of the membrane. At least a portion ofthe absorbed fluorinated urethane polymer exudes from the membrane.

Additional aspects of the invention, together with the advantages andnovel-features appurtenant thereto, will be set forth in part in thedescription which follows, and in part will become apparent to thoseskilled in the art upon examination of the following, or may be learnedfrom the practice of the invention. The objects and advantages of theinvention may be realized and attained by means of the instrumentalitiesand combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the present invention will become apparent to thoseskilled in the art to which the present invention relates from readingthe following description with reference to the accompanying drawings,in which:

FIG. 1 is a schematic view of the process and equipment used to treat amembrane according to the present invention;

FIG. 2 is an enlarged sectional view of a portion of the equipmentillustrated in FIG. 1;

FIG. 3 is an enlarged schematic illustration of a portion of a membranetreated according to the present invention;

FIG. 4 is an enlarged sectional view of a portion of the membrane inFIG. 3 illustrating a coating on the membrane;

FIG. 5 is a graphical representation of various states of a fluid usedin the treatment of the present invention;

FIG. 6 is a graph of the solubility of a treatment material used in thepresent invention at various concentrations; and

FIG. 7 is an SEM photomicrograph of a portion of the membrane treatedaccording to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The present invention includes a method of treating a porous membrane tochange or modify one or more of its properties or characteristics. Thepresent invention also includes the resultant treated membrane having atleast one modified property. The porous membrane may be any suitableporous membrane and is preferably microporous. The membrane may be madefrom any suitable material, such as expanded polytetrafluoroethylene(ePTFE). The treatment may be any suitable treatment that would changeor modify at least one property or characteristic of the porousmembrane, such as, without limitation, color, oleophobicity,hydrophilicity, electrical conductivity, optical reflectivity, ionconductivity, porosity or amount of crystallinity.

There are numerous uses for a porous membrane that has a property orcharacteristic changed or modified. By way of example, a laminatedfabric incorporating a treated or modified composite membrane 12 (FIG.3), made according to the present invention, may be used in garments orapparel. The composite membrane 12 is wind resistant, waterproof,moisture vapor transmissive and air permeable. The composite membrane 12has oleophobicity as the property modified by the treatment method tooffer protection from contaminating agents, such as oil-containing bodyfluids in the form of perspiration.

“Moisture vapor transmissive” is used to describe the passage of watervapor through a structure, such as the composite membrane 12. The term“waterproof” is used to describe that the composite membrane 12 does not“wet” or “wet out” by a challenge liquid, such as water, and preventsthe penetration of a challenge liquid through the membrane. The term“wind resistant” is used to describe the ability of the compositemembrane 12 to prevent air penetration above more than about three (3)cubic feet per minute (CFM) per square foot at a differential pressuredrop 0.5 inches of water but has some air permeability to provideenhanced comfort to someone wearing the laminated fabric. “Airpermeable” is used to describe the ability of the composite membrane 12to permit a relatively small amount, for example less than about three(3) CFM per square foot, of air to pass through it. The term“oleophobic” is used to describe a material that is resistant tocontamination from absorbing oils, greases, soap, detergent or bodyfluids, such as perspiration.

The composite membrane 12 made according to the present inventionincludes an untreated or unmodified membrane 16. The untreated orunmodified membrane 16 is porous, and preferably microporous, with athree-dimensional matrix or lattice type structure of numerous nodes 22interconnected by numerous fibrils 24. The material that the membrane 16is made from may be any suitable material but is preferably made ofexpanded polytetrafluoroethylene (ePTFE) that has preferably been atleast partially sintered. Generally, the size of a fibril 24 that hasbeen at least partially sintered is in the range of about 0.05 micron toabout 0.5 micron in diameter taken in a direction normal to thelongitudinal extent of the fibril.

Surfaces of the nodes 22 and fibrils 24 define numerous interconnectingpores 26 that extend completely through the membrane 16 between oppositemajor side surfaces of the membrane in a tortuous path. Preferably, theaverage size S of the pores 26 in the unmodified membrane 16 issufficient to be deemed microporous, but any pore size may be used inthe present invention. A suitable average size S for the pores 26 in theunmodified membrane 16 may be in the range of 0.01 to 10 microns, andpreferably in the range of 0.1 to 5.0 microns. It is known that a porousePTFE membrane, while having excellent hydrophobic properties, isoleophilic. That is, the material making up the unmodified membrane 16is susceptible to contamination by absorbing oil. Once this occurs thecontaminated regions of the unmodified membrane 16 are considered as“fouled” because the pores 26 can be easily wet by a challenge liquid,such as water, and the membrane is no longer considered waterproof.

Liquid penetration resistance of the fouled unmodified membrane 16 maybe lost if a challenge fluid or liquid can “wet” the membrane. Theunmodified membrane 16 is normally hydrophobic but loses its liquidpenetration resistance when the challenge liquid initially contacts andwets a major side of the membrane and subsequently contacts and wets thesurfaces defining the pores 26 in the membrane. Progressive wetting ofthe surfaces defining the interconnecting pores 26 occurs until theopposite major side of the porous membrane 16 is reached by the wettingor challenge liquid. If the challenge liquid cannot wet the porousmembrane 16, liquid penetration resistance is retained.

The membrane 16 is preferably made by extruding a mixture ofpolytetrafluoroethylene (PTFE) fine powder particles (available fromDuPont under the name TEFLON® fine powder resin) and lubricant. Theextrudate is then calendared. The calendared extrudate is then“expanded” or stretched in at least one and preferably two directions toform the fibrils 24 connecting the nodes 22 in a three-dimensionalmatrix or lattice type of structure. “Expanded” is intended to meansufficiently stretched beyond the elastic limit of the material tointroduce permanent set or elongation to the fibrils 24. The membrane 16is preferably then heated or “sintered” to reduce and minimize residualstress in the membrane material. However, the membrane 16 may beunsintered or partially sintered as is appropriate for the contemplateduse of the membrane.

Other materials and methods can be used to form a suitable membrane 16that has an open pore structure. For example, other suitable materialsthat may be used to form a porous membrane include polyolefin,polyamide, polyester, polysulfone, polyether, acrylic and methacrylicpolymers, polystyrene, polyurethane, polypropylene, polyethylene,cellulosic polymer and combinations thereof. Other suitable methods ofmaking a porous membrane include foaming, skiving or casting any of thesuitable materials.

The ePTFE membrane 16 contains many small interconnected capillary-likepores 26 (FIG. 3) that fluidly communicate with environments adjacent tothe opposite major sides of the membrane. Therefore, the propensity ofthe ePTFE material of the membrane 16 to adsorb a challenge liquid, aswell as whether or not a challenge liquid would be adsorbed into thepores 26, is a function of the surface energy of the challengedmaterial, the surface tension of the liquid, the relative contact anglebetween the liquid and challenged material and the size or effectiveflow area of the capillary-like pores.

One way to prevent entry of the challenge liquid into the pores 26 is tomake the pores extremely small. However, this may be undesirable orimpractical. Another way to prevent or minimize the loss of resistanceto liquid penetration of an ePTFE membrane is to have the surface energyof surfaces of the membrane be lower than the surface tension of thechallenge liquid and the relative contact angle more than 90°. Surfaceenergy and surface tension values are typically given in units ofdynes/cm. Examples of surface energies, relative surface tensions andsome measured relative contact angles are listed in the table below:Contact Material Surface Energy Surface Tension Angle PTFE 18-19dynes/cm deionized water   72 dynes/cm 110°-112° tap water varies with114°-118° source blood   60 dynes/cm 88° perspiration   42 dynes/cmlaundry detergent mix 30.9 dynes/cm 112°  MIBK 23.6 dynes/cm 42° acetone23.5 dynes/cm 37° 100% IPA 20.9 dynes/cm 62° hexane 17.9 dynes/cm 52°DEET 14.8 dynes/cm liquid CO₂ (20° C., 58  1.5 dynes/cm supercriticalCO₂ ≈0.0 dynes/cm

In the course of experimentation it was found that a porous membrane 16could be coated or treated with a modifier, such as a fluorinatedpolymer material in such a way that enhanced oleophobic property resultswithout compromising the air permeability of the membrane. The compositemembrane 12 includes a treatment or coating 28 (FIG. 4) on surfaces ofthe membrane 16. Most significantly the coating 28 adheres and conformsto the surfaces of the nodes 22 and fibrils 24 that define the pores 26in the membrane 16. The coating 28, thus, improves or modifies theoleophobicity of the material of the membrane 16 to resist contaminationfrom absorbing of contaminating materials such as oils, body oils inperspiration, fatty substances, soap, detergent-like surfactants andother contaminating agents. The composite membrane 12 embodying thepresent invention remains durably liquid penetration resistant whensubjected to rubbing, touching, folding, flexing, abrasive contact orlaundering.

The coating 28 adds a relatively low surface energy layer to an ePTFEmembrane so a relative contact angle of most challenge liquids, oils andcontaminating agents is greater than 90° so they cannot foul thecomposite membrane 12. There are several such oleophobic polymericcoatings that appear to be suitable. One example of a suitableoleophobic coating is a fluorinated urethane polymer and is marketed asNRD-342 by DuPont. Most known treatment materials are polymer resinsmade by emulsion polymerization and are sold as aqueous dispersions.These polymers are typically used to treat fabrics as a treatment forcarpets or as a dirt and stain resistance treatment. These treatmentsare typically used on fabric yarns, threads, filaments and fibers thatare significantly larger in size than the nodes 22 and fibrils 24 of themembrane 16. These yarns, threads, filaments and fibers are generallymade from material with a relatively high surface energy that allowaqueous dispersions to wet and ultimately treat the entire yarn, thread,filament or fiber. These yarns, threads, filaments and fibers alsodefine significantly larger voids even in a tightly knit or woven fabricthan the pores 26 in the membrane 16 so there is generally no problemwith coating all surfaces with the particle solids suspended in thewater based dispersion treatment material.

The preferred aqueous dispersion of treatment material containsrelatively low molecular weight fluorinated urethane polymer particlesor “solids”. The dispersion also includes water and surfactant, such assodium dodecyl benzene sulfonate to suspend the particles in the waterand minimize the chance of the solids to form agglomerates. The polymerparticles are preferably separated from the water and the surfactantprior to use according to the present invention. There could besolvents, co-solvents or other surfactants in the dispersion withoutdetracting from the spirit and scope of the present invention. Othersuitable treatment materials that include fluorinated urethane polymerparticles are DuPont's Zonyl® C700 or TLF-9526. Another suitabletreatment material is the Zonyl® family of fluorinated acrylic polymers(made by DuPont and available from CIBA Specialty Chemicals), such asZonyl® 7040. These chemicals are also examples of stain resistanttreatments typically used for carpets, textiles, fibers and fabrics butnot microporous membranes.

Substantially improved oleophobic properties of the microporous membrane16 are realized if the surfaces defining the pores 26 in the membraneand the major side surfaces of the membrane are treated or coated withany of the fluorinated polymers described above, and especially with thepreferred oleophobic fluorinated urethane polymer treatment material.The limiting factor previously has been the lack of an effective way tointroduce the polymer into the pores 26 of the membrane 16 to evenlycoat the surfaces of the nodes 22 and fibrils 24 that define the pores.The present invention provides a way to introduce the polymer into eventhe smallest pores 26 of the membrane 16 to apply a relatively thin andeven coating 28 to the surfaces of the nodes 22 and fibrils 24 thatdefine the pores without having much of an impact on the size of thepores. Furthermore, the present invention provides a way to apply acoating 28 that may modify properties other than oleophobicity of themembrane 16, such as hydrophilicity, electrical conductivity, opticalreflectivity, ion conductivity and color depending on the treatmentmaterial that is used.

It has been found that a fluid under supercritical conditions candissolve the preferred fluorinated urethane polymer particles. Thesolubility of the preferred treatment material in supercritical carbondioxide is illustrated in FIG. 6 at various concentrations. Theresulting solution is capable of wetting the membrane 16 and enteringpores 26 in the microporous membrane 16 with the dissolved fluorinatedurethane polymer. The solution with dissolved fluorinated urethanepolymer has a surface tension, viscosity and relative contact angle thatpermit the dissolved treatment material to be easily carried into thesmallest pores 26 of the membrane 16 with the solvent.

The solvent is preferably carbon dioxide in a supercritical phase asillustrated in FIG. 5. The surface tension of the supercritical carbondioxide (SCCO₂) solution is less than 5.0 dynes/cm, preferably less than1 dyne/cm and most preferably less than 0.1 dyne/cm so it can enter verysmall areas of the article to be treated. Supercritical carbon dioxidealso has a viscosity of less than about 0.1 centipoise. The viscosityand surface tension of the solution are extremely low so very littleresistance to flow is encountered, thus, lending itself to thepossibility of entering even the smallest pores or areas, such asbetween portions of the PTFE molecules of the membrane 16. Thus, it ispossible according to the present invention to enter and coat porousmembrane material with a relatively small pore size that has beenheretofore impossible.

Particularly attractive properties are provided by SCCO2 in that itbehaves like a gas and a liquid at the same time. When it behaves like aliquid, it can dissolve material and act as a solvent as describedabove. The density of SCCO2 is about 0.9 grams/cc so it functions like asolvent. The carbon dioxide is not harmful to the environment since itis preferably obtained from sources that create it as a by-product andcan be repeatedly recovered and re-used. When SCCO2 behaves like a gasit has very low viscosity and surface tension, so it can enter verysmall spaces, such as a relatively small pore in an ePTFE membrane 16 orspaces or voids in a PTFE node 22, fibril 24 or molecule forming themembrane.

The preferred oleophobic fluorinated urethane polymer particles aredeposited onto the surfaces of the nodes 22 and fibrils 24 which definethe pores 26 of microporous membrane 16 to form the coating 28 to reducethe surface energy of the composite membrane 12. The fluorinatedurethane polymer coating 28 of the composite membrane 12 also serves toincrease the contact angle for a challenge liquid relative to thecomposite membrane. Thus, relatively few challenge liquids are capableof wetting the composite membrane 12 and enter the pores 26.

The coating 28 of the present invention is disposed on and aroundsurfaces of the nodes 22 and fibrils 24 that define the interconnectingpores 26 extending through the membrane 16. A small amount of thetreatment material is also absorbed into the material of the membrane16. Once a predetermined proper amount of fluorinated urethane polymerparticles is deposited on the membrane 16, it was found that the pores26 in the composite membrane 12 were not dramatically reduced in flowarea from that of an uncoated membrane. This results in a relativelythin and even coating 28 being applied to the membrane 16.

After the ePTFE membrane 16 is manufactured, the oleophobic fluorinatedurethane polymer is applied to the membrane in such a manner that itenters the pores 26 defined by the surfaces of the nodes 22 and fibrils24. It is not necessary that the coating 28 completely encapsulate theentire surface of a node 22 or fibril 24 or is continuous to increaseoleophobicity of the membrane 16, but it is preferred. The relativelythin coating 28 results from evenly depositing numerous smallfluorinated urethane polymer particles on as much of the surface area ofthe membrane 16 as possible, including surfaces defining the pores 26.

The size of the precipitated particle is believed to be in the range ofabout 1.0 nanometer to about 10.0 nanometers in diameter and preferablyin the 1.0 nanometer to 5.0 nanometers range. It is believed that theparticle size that is precipitated depends on the rate ofdepressurization. Thus, the ratio of the deposited coating 28 thicknessT2 to the fibril 22 size T1 is in the range of 0.2% to 20% and for thepreferred particle size the range is 0.2% to 10%. The ratio of thedeposited coating thickness T2 to the effective average size S of thepores 26 is in the range of 0.2% to 10% and for the preferred particlesize the range is 0.2% to 5%.

The fluorinated urethane polymer particles engage and adhere to surfacesof the nodes 22 and fibrils 24 that define the pores 26 in the membrane16 after the particles precipitate out of the solvent. The depositedfluorinated urethane polymer particles may be heated on the membrane 16to flow and cover the surfaces of the nodes 22 and fibrils 24 andthereby render the composite membrane 12 even more resistant tocontamination from absorbing oils and contaminating agents. During theapplication of heat, the thermal mobility of the fluorinated urethanepolymer particles orients the —CF3 groups contained in the polymer onthe nodes 22 and fibrils 24. The —CF3 groups of the preferred polymerorient to extend into the air to better repel challenge liquids. Thefluorinated urethane polymer coating 28, thus, provides a relativelythin and maximized protective coating on the membrane 16 that does notcompletely block or “blind” the pores 26 in the composite membrane 12,as illustrated in FIG. 7, that could adversely affect moisture vaportransmission or air permeability through the composite membrane.

The composite membrane 12 of the present invention has a relatively highmoisture vapor transmission rate (MVTR) and air permeability while itsoleophobic properties are improved by the coating 28. The compositemembrane 12 has an oil hold out of at least a number 6 and preferably isa number 8 as determined by AATCC 118 testing. In some cases, theoleophobicity can be further improved by heating the deposited materialthat forms the coating 28. The composite membrane 12 preferably has amoisture vapor transmission rate (MVTR) of at least 50,000 g/m2/day andmore preferably at least 70,000 g/m2/day measured by JIS-1099B2 testing.The composite membrane 12 is air permeable to a sufficient degree that auser of apparel made from the composite membrane can be relativelycomfortable in most conditions and even during periods of extremephysical activity. The composite membrane 12 preferably has anair-permeability of at least 0.20 CFM per square foot of membrane andmore preferably at least 0.30 CFM per square foot of membrane measuredby ASTM D737 testing.

The composite membrane 12 has at least a portion of the fluorinatedurethane polymer treatment material forming the coating 28 absorbed intothe material of the membrane 16. That is, portions such as molecules ofthe fluorinated urethane polymer treatment material enter small regionsin the PTFE material of the membrane 16. The portions of the treatmentmaterial are engaged by at least two amorphous portions of the membrane16 to mechanically capture and at least partially encapsulate some ofthe material of the coating. Thus, the treatment material of the coating28 is more difficult to wash out or be removed by abrasion or flexing ofthe composite membrane 12. If some of the coating 28 is washed away orremoved by damage or attrition, the coating is repaired by absorbedtreatment material exuding from the PTFE.

The treatment material of the coating 28 is absorbed by spaced apartamorphous portions of PTFE of molecule when the PTFE membrane materialswells as it is exposed to supercritical carbon dioxide. The PTFEmaterial may swell up to about 30 percent of its initial size whenexposed to supercritical carbon dioxide. The low viscosity and lowsurface tension solution carries the treatment material polymer intoextremely small voids within of the PTFE material. When the carbondioxide transitions to a condition outside its supercritical phase, thePTFE material is no longer swelled. Any portions or molecules of thefluorinated urethane polymer surrounded by the swelled portions of thePTFE can be mechanically engaged or trapped by the now unswelled PTFEmaterial of the membrane 16. At least a portion of the absorbedfluorinated urethane polymer can exude from the membrane. This exudingprocess is a self-healing mechanism that maintains the oleophobicproperties of the composite membrane 12 for a relatively long period oftime by replacing missing or damaged portions of the coating 28. Exudingof the captured portions of the coating 28 inherently occurs over timebut is accelerated when the composite membrane 12 is exposed to heat orultraviolet light, such as sunlight. Heat and sunlight provide energy tovibrate the PTFE material. The vibration allows the absorbed material toovercome the attractive force holding it in the PTFE material and moveor exude from its original location inside the PTFE material to theouter surface.

The solution or even supercritical carbon dioxide on its own can also beused to break the crystalline bonds between portions of the PTFEmolecule of the membrane 16. Thus, sintering can be performed withoutheat. This is accomplished by adjacent crystalline portions of the PTFEmaterial being forced apart due to swelling when exposed tosupercritical carbon dioxide. The distance separating these swelledadjacent portions of the PTFE molecule exceeds the distance required byVan der Waals forces to maintain molecular crystallinity. Thus, thisseparation becomes permanent and a more amorphous ePTFE membraneresults.

System Equipment

Equipment 60 for use in the method of treating the membrane 16 accordingto the present invention is schematically illustrated in FIG. 1. Labscale equipment, based on the equipment 60, was used in most of theexamples described below. The equipment 60 includes a treatment vessel62 for treating the membrane 16. The treatment vessel 62 is preferablyin the form of an autoclave capable of withstanding pressure up to10,000 psi (about 690 bar) and elevated temperature in the range of 100°C. to 300° C. (212° F. to 572° F.). The treatment vessel 62 is sizedappropriately to treat the desired width and length of membrane 16. Thetreatment vessel 62 is fluidly connected to a supply and circulationpump 64 by line 66. The treatment vessel 62 has an external heater 68 tomaintain the walls of the treatment vessel at a predeterminedtemperature. The treatment vessel 62 is located in a fluid circulationloop connected by line 82 to a temperature control device 84, optionalstatic mixer 86 and treatment introduction vessel 88. The treatmentintroduction vessel 88 is connected to pump 64 through line 102 andvalve 104. Valve 104 and valve 106 allow flow through line 108 to bypassthe treatment introduction vessel 88. The temperature control device 84may provide cooling or heating to the line 82 and the fluid contained inthe line. Any or all of the lines and vessels may be heated or cooled tocompensate for cooling when the CO2 expands or heating when the CO2 iscompressed.

Pump 64 is also connected to a solvent storage container 122 throughline 124 and valve 126. The storage container 122 houses liquid solventunder pressure and is maintained at a temperature to assure delivery ofsolvent in a liquid phase to pump 64. The treatment vessel 62 is alsoconnected to separation and recovery station 142 through line 144 andvalve 146. The separation and recovery station 142 is vented toatmosphere or may be optionally connected to the storage container 122for reusing recovered CO2.

The untreated membrane 16 is rolled onto a core 180, as illustrated inFIG. 2, and the ends of the roll secured with securement mechanisms 64such as clamps to hold the membrane on the core and prevent fluid flowaxially out the ends of the roll. The securement mechanisms 64 arepreferably radially and circumferentially contractible. The securementmechanisms 64 are sufficiently tightened so no fluid flows in adirection axially out the ends of the roll of membrane 16 betweenradially adjacent wraps but radially through the pores 62 in every wrapof the roll of membrane as indicated by arrows F. The core 180 is madefrom any suitable material, such as perforated stainless steel andincludes a multiplicity of openings 204 extending radially through thecore. The core 180 and membrane 16 are supported in the treatment vessel62 so the membrane 16 does not contact the interior of the treatmentvessel 62 and fluid flow can occur around the entire roll of membrane.

While any suitable connection, support and cap structure may be used,the core 180 is sealed at one axial end to a core cap 182 a that iswelded to the core. The core cap 182 a is attached to a removablysecurable end cap 184 of the treatment vessel 62 by a threadedconnection 182 b. The core 180 is shown extending horizontally. It willbe apparent that the core 180 and treatment vessel 62 could be orientedin a vertical direction or any other orientation. The interior of thecore cap 182 a, threaded connection 182 b and core 180 are in fluidcommunication with line 82 through a port in the end cap 184.

The other axial end of the core 180 has a second removably securablecore cap 202 that prevents fluid flow out that end of the core. Thenumerous openings 204 in the core 180 direct fluid to flow radially fromthe inside the core, through the pores 26 in all the layers in the rollof membrane 16 and into a space 206 (FIG. 1) between the exterior of theroll of membrane and the interior wall 208 of the treatment vessel 62 asindicated by arrows F (FIG. 2). In operation, a pressure differential ofabout 30 psi was observed between the inside of the core 180 and theoutside of the roll of membrane 16. It will be apparent that thepressure differential may vary and is a function of fluid flow velocity,roll size, pore size and pore density. Fluid flows from the space 206(FIG. 1) in the treatment vessel 62 through an opening in a secondremovably securable end cap 212 of the treatment vessel 62 through aport and to line 66.

Process

The treatment material may require separation of the polymer particlesolids from the dispersion that it is available in. Particle solids ofthe preferred fluorinated urethane polymer treatment material are placedin the treatment introduction vessel 88. The amount of treatmentmaterial depends on the solution concentration desired in the system.The core 180 and roll of membrane 16 are placed in the treatment vessel62 and connected by the threaded connection 182 b to end cap 184 forfluid flow through the core and roll. End caps 184 and 212 are securedto seal the treatment vessel 62. The membrane 16 is made from a materialthat does not dissolve in the selected fluid solvent. Vacuum is appliedto the system and maintained for sufficient time to remove generallyundesired substances like water and air.

Valve 146 is closed and valve 126 is positioned to allow fluid flow tothe system. Liquid solvent, such as the preferred carbon dioxide, flowsfrom the storage container 122 into the treatment vessel 62 and the restof the system at the storage pressure. Valves 104 and 106 are initiallypositioned to bypass vessel 88 and create a closed circulation loopbetween the treatment vessel 62 and pump 64. Pump 64 is started to fillall lines 102, 108, 82 and 66, vessel 62 and mixer 86 and increasepressure. Valve 126 is positioned to block flow from container 122 andpermit flow between the pump 64 and treatment vessel 62. Pump 64 raisesthe pressure in the system to a predetermined pressure. Valves 104 and106 are positioned to close off bypass line 108. Fluid flows from thepump 64, through line 102, treatment introduction vessel 88, staticmixer 86, line 82 and to treatment vessel 62.

System pressure increases to a desired predetermined pressure. Thetemperature and pressure of the solvent is controlled as determined bythe solubility of the treatment material to be in a phase or conditionso the treatment material may dissolve, as illustrated in FIG. 6, for adesired solute concentration. Pressure and volume of solvent may beincreased in a known manner by a make-up supply and pump (not shown).

It has been found that particularly suitable treatment materials areNRD-342 and Zonyl® C700. The treatment material is exposed to the fluidwhen the fluid is in a phase that can solubilize the treatment material.One such fluid solvent is carbon dioxide in a supercritical phase (FIG.5). For example, when supercritical carbon dioxide (SCCO2) is at 220 baror higher pressure and a temperature of 35° C., as illustrated in FIG. 6for the concentration of up to 4%, the preferred treatment materialNRD-342 particles dissolve in the solvent. Each concentration line inFIG. 6 represents a “cloud point” where the solute visually becomesinsoluble and begins to precipitate out of the supercritical fluidduring a phase monitor study as a function of pressure. The treatmentmaterial solid particles in the treatment introduction vessel 88dissolve in the solvent flowing through it at supercritical conditions.

Other treatment materials can be used and have their own solubilityparameters that can be determined in phase monitor studies. It will beapparent that any suitable fluid capable of becoming supercritical canbe used and the use of a co-solvent such as methyl isobutyl ketone(MIBK) may be desired. Flow through the vessel 88 continues until thedesired concentration of the treatment material solute in the solvent isattained. It will also be apparent that the treatment material can be inliquid form and pumped into the system. It may be desirable to equalizepressure between the interior of the core 180 and the exterior 206 ofthe roll by apparatus not shown until certain system conditions, such asconcentration, or pressure and/or temperature are reached. This flowpath is maintained until the desired amount of solids in the treatmentintroduction vessel 88 is dissolved to obtain a desired predeterminedconcentration of treatment material in the solution.

Once the desired system conditions are reached, the treatment materialsolute and solvent in the solution are circulated through the system foran appropriate predetermined time. The flow path may be any suitableflow path. By way of example, the solution is routed through the pump64, treatment introduction vessel 88 (or bypassed through line 108),static mixer 86, temperature control device 84, line 82, through end cap184, into the interior of the core 180, through the pores 26 in the rollof membrane 16, into the space 206 in the treatment vessel 62, throughthe cap 212, through line 66 and then back to pump 64. This assures thatevery pore 26 in the roll of membrane 16 has been exposed to thetreatment material. For the NRD-342 and Zonyl® C700 treatment materials,a solution concentration in the range of 1 weight percent to 5 weightpercent in the supercritical carbon dioxide solvent was used and foundto be suitable.

After the desired concentration of treatment chemical is obtained in thesolution, the solution is circulated in the closed loop system for apredetermined time to assure that every pore 26 in every layer of theroll of the membrane 16 has the treatment material at the desiredconcentration of treatment material flowing through it. The solution ofthe treatment material is circulated through the treatment vessel byentering the cap 184 at a central location. The end cap 184 has the core180 attached by connection 182 b (FIG. 2). The solution of treatmentmaterial flows through the core 180, through all the pores 26 in theroll of membrane 16 and into the space 206 between the roll of membraneand the interior wall 208 of the treatment vessel 62. The solution ofthe treatment material then flows though a port in the end cap 212 andinto line 66. When the solution circulates for sufficient time at thedesired conditions, the pump 64 is stopped. Enough time is allowed tolapse to assure that the fluid has stopped moving in the system andparticularly in the pores 26 in the membrane 16 due to its momentum tominimize the chance that treatment material can be carried away from thepores with flow.

The pressure and/or temperature of the solution are/is then permitted tochange to a condition in which the treatment material solute is nolonger soluble, as illustrated in FIG. 6. For example, the pressure isreduced to 150 bar and the temperature is maintained at 35° C. Thepressure can then be further reduced to atmospheric so the treatmentvessel 62 can be opened. If the treatment material is soluble in liquidcarbon dioxide, the temperature and pressure are controlled to keep thecarbon dioxide in the gaseous state during emptying of the treatmentvessel 62.

The treatment material precipitates out of the solution when it firstbecomes insoluble. The precipitated treatment material deposits onto thesurfaces of the nodes 22 and fibrils 24 defining the pores in the porousmembrane 16 to form the coating 28 (FIGS. 3 and 4). The coating 28 oftreatment material is extremely thin and evenly distributed on thesurface defining the pores 26 of the membrane 16. The depositedtreatment material coating 28 does not block the pores 26 of themembrane 16 so air permeability of the membrane is not adverselyaffected. The particle size of the deposited treatment material is about1-5 nanometers. The size of the particle precipitated can be increasedby depressurizing slower. The deposited treatment material covers all orat least substantially all of the surface area of the membrane 16.

At least a portion of the fluorinated urethane polymer is absorbed intoamorphous portions of the membrane 16. This occurs because amorphousportions of the PTFE membrane material swell as much as 30 percent fromtheir initial unswelled size. When the supercritical carbon dioxidesolvent changes from its supercritical phase to subcritical, the PTFEmaterial returns to its initial size and portions of the depositedpolymer treatment material are mechanically encapsulated or “captured”by the PTFE material of the membrane 16. At least a portion of theabsorbed fluorinated urethane polymer may exude from the membrane withtime and is accelerated by exposure to heat or to sunlight.

The membrane modifying method may include exposing the ePTFE membrane tojust carbon dioxide (CO₂) at supercritical conditions to swell a portionof the ePTFE membrane from an initial size to a swelled size. That is,no treatment material is used. The crystalline bonds in the swelledportion of the ePTFE membrane are broken to render the swelled portionmore amorphous. The ePTFE membrane is removed from exposure of thecarbon dioxide (CO₂) at supercritical conditions. The portion of theePTFE membrane returns towards the initial size while retaining theamorphous condition in that portion of the ePTFE membrane. DSC resultsconfirm that there was an increase in amorphous content.

Post-Treatment Heat

Heat may optionally be applied to the composite membrane 12 with theprecipitated coating 28 applied. Heat may be applied at about 140° C.heat for about thirty (30) seconds to the composite membrane 12. Theapplied heat permits the coating 28, such as the fluorinated urethanepolymer solids precipitated onto the membrane 16, to further flow aroundthe surfaces of the nodes 22 and fibrils 24 to become even moreuniformly distributed and thinner to render the composite membrane 12oil and contaminating agent resistant to a more significant degree thana composite membrane that has not been heated. The heat that is appliedto the composite membrane 12 accelerates the fluorine portions (notshown) orienting to extend in a direction away from the surfaces of thenodes 22 and fibrils 24 that are coated.

EXAMPLE 1

Approximately 60 yards of ePTFE membrane 16 (QM011SP available from BHATechnologies, Inc. in Kansas City, Mo.) was wound onto a three inchoutside diameter perforated core 180 in about 200 wraps. The roll ofmembrane 16 had an outside diameter of about 3.95 inches and a distancebetween the clamps of 22.3 inches. The average effective pore size ofthe membrane 16 was about 0.4 micron. 600 ml of TLF-9526 treatmentmaterial was placed in the treatment introduction vessel 88. A syringepump was connected to the treatment introduction vessel 88 and one ofthe circulation lines in the system. The treatment material wasintroduced into a system volume of about 13 liters of supercritical CO2flowing at a rate of about 1500 grams/minute at 300 bar and 40° C. bythe pump 64. The treatment material solution was circulated in thesystem and flowed through the core 180 and membrane 16. The treatmentmaterial solution was circulated for one hour and the system wasdepressurized slowly. The membrane 12 removed from the treatment vessel62 and core 180. The treated membrane 12 was tested. The results arereported in the table below. oil hold out air permeability MVTR rolllocation side 1 side 2 side 1 side 2 side 1 side 2 end 1 6 6 0.30 0.2586000 85000 middle 6 6 0.25 0.34 83000 86000 end 2 7 6 0.39 0.32 8600086000

EXAMPLE 2

Approximately 70 yards of ePTFE membrane 16 (QM011available from BHATechnologies, Inc. in Kansas City, Mo.) was wound onto the perforatedcore 180. The average effective pore size of the membrane 16 was about0.5 micron. 284 grams of TLF-9526 solids treatment material was placedin a treatment introduction vessel 88 with frits on each end. Thetreatment material solids were dissolved by about 13 liters ofsupercritical CO₂ flowing through the vessel 88. The treatment materialsolution was circulated in the system and flowed through the core 180and membrane 16 in both directions for forty-five minutes. The systemwas then depressurized quickly. The membrane 12 removed from thetreatment vessel 62 and core 180. The treated membrane 12 was tested.The results are reported in the table below. oil hold out airpermeability MVTR roll location side 1 side 2 side 1 side 2 side 1 side2 end 1 8 8 0.39 0.39 81000 69000 middle 7 7 0.38 0.37 79000 66000 end 28 8 0.36 0.38 73000 69000

EXAMPLE 3

Several trials were performed by exposing the membrane 16 only to CO₂ atsupercritical conditions. This was to determine the effects of exposureto SCCO2. One trial was performed by exposing the membrane 16 to SCCO2at 280° C. at 4000 psi for 60 minutes. The membrane 12 showed only aslight decrease in Joules/gram by DSC analysis. A control sample of themembrane had 62.57 J/gram before exposure to SCCO2 and a membrane sampleafter exposure to SCCO2 measured 60.45 Joules/gram. Another trial wasconducted by exposing a membrane 16 to SCCO2 at 327° C. at 4000 psi for60 minutes. The pressure reduction was done at a rate of 5 psi/minutefrom 4000 psi to 1000 psi, and then from a 1000 psi to atmosphericpressure over a 60 minute period. A control sample of this secondmembrane had 47.63 J/gram before exposure to SCCO2 and the secondmembrane 12 sample after exposure to SCCO2 measured 27.23 J/gram. Yetanother trial was conducted by exposing a membrane 16 to SCCO2 at 330°C. and 4000 psi for 60 minutes. A control sample of this third membranesample had 57.06 J/gram before exposure to SCCO2 and the third membrane12 sample after exposure to SCCO2 measured 30.86 J/gram.

EXAMPLE 4

Approximately 130 yards of ePTFE membrane 16 was wound onto theperforated core 180. The average effective pore size of the membrane 16was about 0.5 micron. 400 grams of TBCU-A solids treatment material wasplaced in the treatment introduction vessel 88 with frits on each end.The treatment material was dissolved by about 13 liters of supercriticalCO₂ flowing through the treatment introduction vessel 88. The treatmentmaterial solution was circulated in the system at a rate of about 1600grams/minute at 225 bar at an average temperature of 40° C. Thetreatment material solution flowed through the core 180 and membrane 16from outside the roll and core to inside the core for thirty minutes andfrom inside the core to outside the core and roll for thirty minutes.The system was then depressurized in a controlled manner to keep the CO2in a gaseous state until pressure reached 800 PSI. Fast depressurizationwas then permitted. The membrane 12 removed from the treatment vessel 62and core 180. The treated membrane 12 was tested. The results arereported in the table below. oil hold out air permeability MVTR rolllocation side 1 side 2 side 1 side 2 side 1 side 2 end 1 6 6 0.38 0.3174000 89000 middle 6 6 0.32 0.28 84000 82000 end 2 6 6 0.30 0.24 8400069000

EXAMPLE 5

Approximately 130 yards of ePTFE membrane 16 was wound onto theperforated core 180. The average effective pore size of the membrane 16was about 0.5 micron. 488 grams of NRD-342 solids treatment material wasplaced in the vessel 88 with frits on each end. The treatment materialwas dissolved by about 13 liters of supercritical CO₂ flowing throughthe system at a rate of about 1600 grams/minute at 280 bar and anaverage temperature of 37° C. The treatment material solution wascirculated in the system and flowed through the core 180 and membrane 16from the inside to the outside of the roll of membrane for thirty-fourminutes. The system was then depressurized in a controlled manner tokeep the CO2 in a gaseous state until pressure reached 800 PSI. Fastdepressurization of the treatment vessel was then permitted. Themembrane 12 removed from the treatment vessel 62 and core 180. Thetreated membrane 12 was tested. The results are reported in the tablebelow. oil hold out air permeability MVTR roll location side 1 side 2side 1 side 2 side 1 side 2 end 1 8 8 0.32 0.31 90000 84000 middle 8 80.27 0.28 77000 86000 end 2 8 8 0.24 0.24 79000 72000

EXAMPLE 6

Vacuum was applied to the system initially. Approximately 130 yards ofePTFE membrane 16 was wound onto the perforated core 180. The averageeffective pore size of the membrane 16 was about 0.229 micron. 400 gramsof NRD-342 solids treatment material was placed in the vessel 88 withfrits on each end. The treatment material was dissolved by about 13liters of supercritical CO2 flowing through the system at a rate ofabout 1600 grams/minute at 280 bar and an average temperature of 36° C.The treatment material solution was circulated in the system and flowedthrough the core 180 and membrane 16 from the inside to the outside ofthe roll of membrane for thirty-four minutes. The system was thendepressurized in a controlled manner to keep the CO2 in a gaseous stateuntil pressure reached 800 PSI. Fast depressurization of the treatmentvessel 62 was then permitted. The membrane 12 removed from the treatmentvessel 62 and core 180. The treated membrane 12 was tested. The resultsare reported in the table below. oil hold out air permeability MVTR rolllocation side 1 side 2 side 1 side 2 side 1 side 2 end 1 8 8 0.23 0.2176000 67000 middle 8 8 0.22 0.20 62000 49000 end 2 8 8 0.21 0.18 6600068000

EXAMPLE 7

To determine if the fluorinated urethane polymer was captured by theePTFE membrane by the following procedure. A sample (approximately5″×5″) of membrane 12 that initially showed a number 8 oil holdout fromExample 5 above was selected. This sample was soaked in methyl isobutylketone (MIBK) for a few minutes. The sample was removed from the MIBKand the surface was wiped with a paper towel. The sample was soaked inabout fresh MIBK. The sample was removed from the MIBK and the surfacewas wiped with a paper towel. This procedure should ensure that all thefluorinated urethane polymer is removed from all the surfaces of themembrane 12. The sample was air dried overnight and the oil holdoutmeasured at a number 4 by the AATCC 118 test. The sample was thenexposed to sunlight for two days and the oil holdout measured at anumber 5. After heating the sample, the oil holdout remained at a number5.

Because untreated ePTFE membrane 16 has an oil holdout number of 1, anoil hold of at least a number 2 suggests that the fluorinated urethanepolymer was mechanically captured by the ePTFE membrane during thetreatment process of the present invention. The increase in oil holdoutafter exposure to sunlight suggests that the fluorinated urethanepolymer does exude.

EXAMPLE 8

A wash durability test was conducted to further determine if thefluorinated urethane polymer was captured by the ePTFE membrane 12. Asample of membrane 12 that initially showed a number 8 oil holdout fromExample 5 above was selected. The sample was sewn into a protectiveshell. Wash water temperature was 80° F. without soap. The samples weredried at 150° F. before each oil holdout test. The results of the testare shown below and suggest that the fluorinated urethane polymer isdurable on the membrane 12. Wash hours Oil holdout  0 8 15 8 25 8 35 850 8

EXAMPLE 9

Approximately 249 yards of ePTFE membrane 16 was wound onto theperforated core 180. The average effective pore size of the membrane 16was about 0.5 micron. The roll of membrane 16 had an outside diameter ofabout 6.4 inches and a distance between the clamps of about 65 inches.4005 grams of NRD-342 dried solids treatment material was placed in thetreatment introduction vessel 88 between frits. The treatment materialwas dissolved by about 105,000 grams of supercritical CO2 flowingthrough the treatment introduction vessel 88. The treatment materialsolution was circulated in a relatively larger scale system thanprevious treatment examples at a rate of about 2700 grams/minute at 260bar at an average temperature of 32° C. The treatment material solutionflowed through the core 180 and membrane 16 from inside the core tooutside the core and roll for ninety minutes. The system was thendepressurized in a controlled manner to keep the CO2 in a gaseous state.The membrane 12 removed from the treatment vessel 62 and core 180. Thetreated membrane 12 was tested. The results are reported in the tablebelow. oil hold out air permeability MVTR roll location side 1 side 2side 1 side 2 side 1 side 2 end 1 8 8 0.35 0.35 85000 89000 middle 8 80.35 0.38 89000 88000 end 2 8 8 0.32 0.28 92000 73000

From the above description of preferred embodiments of the invention,those skilled in the art will perceive improvements, changes andmodifications. Such improvements, changes and modifications within theskill of the art are intended to be covered by the appended claims.

1. Sheet material comprising: a porous membrane made from expandedpolytetrafluoroethylene (ePTFE) and having a structure of nodesconnected by fibrils in which surfaces of said nodes and fibrils definea plurality of interconnecting pores extending through said membrane andbetween major sides of said membrane; said membrane material havingspaced apart portions; and precipitated material on portions of thesurfaces of said nodes and fibrils defining the pores without completelyblocking the pores in said membrane; said precipitated material on saidsurfaces of said membrane provides a change in at least one property ofsaid membrane; portions of said precipitated material being absorbed byportions of the PTFE material of said ePTFE membrane.
 2. The sheetmaterial of claim 1 further including post treatment of the precipitatedmaterial providing is oleophobic properties to said membrane.
 3. Thesheet material of claim 1 wherein said one property of the sheetmaterial that is modified selected from the group of oleophobicity,hydrophilicity, electrical conductivity, optical reflectivity, ionconductivity, and color.
 4. The sheet material of claim 1 wherein saidportions of said precipitated material absorbed in the PTFE materialexudes from the PTFE material.